1. Field of the Invention
The present invention relates generally to a control device for an automatic transmission, and more particularly to a control device for an automatic transmission for changing a target slip ratio of a lock-up clutch according to a running condition.
2. Description of the Related Art
In a general automatic transmission for a vehicle, a hydraulic lock-up clutch capable of mechanically directly connecting an output shaft of an engine and an input shaft of the automatic transmission is provided in a torque converter, wherein the lock-up clutch is engaged under given conditions to thereby improve the efficiency of the torque converter. Further, the lock-up clutch is engaged at deceleration to thereby transmit a back drive force from drive wheels to the engine, so that a sudden decrease in engine speed can be suppressed. Accordingly, fuel cut to be performed until reaching a predetermined fuel supply restoring engine speed at deceleration can be continued for a long time to thereby improve the fuel economy.
Usually, the lock-up clutch in the automatic transmission is engaged at a predetermined gear position for the purposes of the improvement in fuel economy, etc. To absorb the shock due to complete engagement of the lock-up clutch, a control method for the lock-up clutch for controlling the slip amount of the lock-up clutch according to an operational condition is generally adopted. For example, a target slip ratio of the lock-up clutch according to an operational condition and a gear position is preliminarily stored in a memory, and the engagement capacity of the lock-up clutch is controlled so that an actual slip ratio of the lock-up clutch becomes the target slip ratio.
In a control device for such an automatic transmission in the prior art, the target slip ratio of the lock-up clutch is changed according to a gear position and according to whether an air conditioner is on or off during running both in the condition where an accelerator pedal is depressed and in the condition where the accelerator pedal is undepressed. However, the target slip ratio is not changed on an uphill road and a level road. When the running road is determined to be an uphill road, a lock-up clutch region where the lock-up clutch is engaged is limited to a high vehicle speed region, hereby substantially disengaging the lock-up clutch. Further, in an automatic transmission having a D range and a manual mode range as a running range, the target slip ratio of the lock-up clutch is not changed irrespective of whether the running range is a D range or a manual mode range, but only the lock-up clutch region is changed.
During running on an uphill road, a running resistance increases and acceleration therefore becomes harder than that on a level road. Accordingly, in the case of running on an uphill road with a target slip ratio for a level road, steady heat generation from the lock-up clutch becomes excessive, causing a possibility of reduction in durability. To cope with this problem, the lock-up clutch is disengaged at acceleration on an uphill road. As a result, the fuel economy on an uphill road is deteriorated, and the drivability is also deteriorated.